Boost.System no longer has a library.

--- f98e72e4963d9c9da57620595282e58d46a045f7/libtrellis/CMakeLists.txt	2025-01-12 16:29:31.000000000 +0000
+++ f98e72e4963d9c9da57620595282e58d46a045f7/libtrellis/CMakeLists.txt	2025-08-15 03:52:23.698824966 +0100
@@ -46,7 +46,7 @@
     endif()
 endif()
 
-set(boost_libs filesystem program_options system)
+set(boost_libs filesystem program_options)
 if (Threads_FOUND)
     list(APPEND boost_libs thread)
 else()